A) A Method of Producing Metallic Parts of Near-Finish Shape.
Metal powder injection molding (MIM) is a method of the mass production of metallic parts, in particular for the production of such parts of near net shape (NNS). The MIM process makes it possible to automatically and inexpensively produce small to medium-large parts in large quantities. The MIM method produces parts with a density equal to 95 to 98% of the theoretical density that can be produced by subsequent hot isostatic compression of the part (without encapsulation).
The method entails the plastification of metal powders with spherical or irregular morphology (particle size from 5 to 300 μm) with a binder system to a so-called feedstock. The goal of the preparation is the coating of all the particles with the organic binder. The feedstock is homogenized in a mixer. Then the feedstock is charged into the injection-molding machine. In a heated zone components of the binder system (e.g. specific waxes) are melted. A worm advances the thermoplastic mass into the separable mold. After the mold is filled, the liquid mass hardens so the parts can be demolded. The binder system is separated out in a binder-removing step before the sintering. According to the type of binder, additives of different types are also stripped from the part.
There are differences between thermal binder-removing systems (melting out or destruction via the gas phase), solvent extraction, and catalytic solvent-stripping methods. Afterward there is the sintering process in which as a result of diffusion the part is densified up to about 98% of the theoretical density. As a result of the considerable binder content, during sintering there is considerable shrinkage (15 to 20% by volume). Controlling the shrinkage is essential in the production of near-net-shape parts.
Typically used materials for the metallic components in metal-powder injection molding are stainless steel, carbon steel, tool steal, or alloy steel, also ferrite, tungsten carbide, and copper/bronze, cobalt/chromium and tungsten/copper mixtures.
B) Method of Making Near-Net-Shape Ceramic Parts.
The known metal powder injection molding (MIM) method has also been applied to the production of ceramic parts. The so-called powder injection molding (PIM) method also can produce ceramic parts as ceramic injection molding (CIM). To get a corresponding injectable powder mass, organic binder is mixed with ceramic powder. The injection molding process and the sintering are carried out as in metal-powder injection molding, taking into account the specific characteristics of ceramic powders (e.g. smaller particle size of the starting powder).
C) Manufacture of Porous Metallic Parts.
The compression of metal powders to produce porous metal bodies is also known from the literature. To achieve the desired porosity the metal powders can be supplemented with so-called place-holding materials that make it possible to achieve the desired porosity. After compressing the green powder-mixture bodies the place-holding material has to be stripped out of the green bodies so that the green bodies hold nothing but the remaining metal-powder lattice forming empty spaces. The green body thus already has the later porous structure of the shaped body. When the place-holding material is driven out, care is taken to ensure that the metal-powder lattice remains. The subsequent sintering of the green bodies produces a highly porous shaped body, the contacting surfaces of the particles being diffused together when sintered.
As place-holding material for forming the porous metallic shaped bodies one uses high-melting-point organic compositions which are vaporized or pyrolized (cracked) with release of the thus produced crack byproducts by means of appropriate solvents from the green bodies. The problem with this is the considerable time it takes to separate the place-holding material and cracking byproducts that react with nearly all the powder-metallurgical metals such as Ti, Al, Fe, Cr, Ni, and the like and that leave behind high concentrations of impurities. This disadvantage is also encountered when thermoplastics are used, because they have to be heated to get them out of the green body so that the expansion at the glass-transition point is bad for the desired stability of the green body.
In addition the place-holding materials can be inorganic high-melting-point materials such as alkali salts and low-melting metals such as Mg, Sn, Pb, and the like. Such place-holding materials are stripped out in a vacuum or in an inert gas at temperatures of about 600 to 1000° C. at considerable cost in terms of energy and time. It is impossible to avoid the harm done by these place-holding materials in particular when used in reactive metal powders such as Ti, Al, Fe, Cr, and Ni.
With alkaline salts there is also the possibility of stripping them out by dissolving them with an appropriate solvent (e.g. water). This method is not ideal for pressed mixtures of metal powders and alkaline salts since the structural integrity of the pressed product is largely lost in the process.
German 196 38 927 describes a method of making highly porous metallic molded bodies wherein first metal powder and a place holder are mixed and then pressed into a green body. Uniaxial and isostatic compression can both be used. The place holder is driven out by heat and the green body is then sintered. If the powder/place-holder mixture is stabilized with a binder, it is usually possible to directly produce a relatively complicated shape in the finished part by multiaxial pressing. The production of a pressing tool to do this is however expensive and difficult. For small series it is thus advantageous to produce intermediate products with a universal shape (e.g. cylinder or plate) and to transform them by a subsequent mechanical step into the desired end shape.
In addition it is necessary to manufacture highly porous parts in large numbers as for use in medicine, air or space travel, or even as filters. Porous parts are made today for example by foaming aluminum or by powder technologies by the use of appropriate place holders. These methods only limitedly allow a near-end shape to be produced of complexly contoured parts in large numbers.